Released in 1992, and developed by Konami, the game came out at the height of the studio's arcade power, during which similar side-scrolling games (all based on licensed properties) like The Simpsons and TMNT took the world by storm.
What made X-Men so special wasn't the game so much as the way the game was presented. Unlike TMNT and The Simpsons, which supported four players, X-Men allowed for six people to play together on the one machine, a move that took some fancy engineering.
Not only did the game require an extra-large cabinet, so as to fit six control layouts on the one board, but it also used two CRT monitors instead of the usual one, so that there was enough screen real estate for all six players to see what was going on.
Rather than being slung up next to each other, which would be a strain on the frame, the two screens were arrayed atop each other, the top one appearing normally, the other one hidden below and reflecting its display via a mirror to give a widescreen effect.
You can take a look at one such example of the monstrosity, obtained by SuperGunGuru for a handy $175, below.
Re: Obtained an X-Men 6 Player cab! [Arcade Controls, thanks Shannon!]